1. Field
The present disclosure relates to a control device for an internal combustion engine.
2. Description of the Related Art
There has been known a conventional EGR ratio estimating method which involves detecting an amount of fresh air to be taken into a cylinder by an airflow sensor, calculating a total amount of gas to be taken into the cylinder based on an intake pressure detected by an intake pressure sensor, and estimating an EGR ratio from the amount of fresh air and the total amount of gas. In the case of using this estimating method for a low-pressure EGR device (EGR device which takes in an exhaust gas from a downstream side of a turbine of a turbocharger and recirculates the exhaust gas back to an upstream side of a compressor in an intake passage), however, it is difficult to estimate the EGR ratio with high accuracy because the external EGR gas needs to flow a relatively long passage leading to the cylinder, and accordingly reaches the inside of the cylinder with a long time lag.
As another conventional EGR ratio estimating method, a method disclosed in Japanese Unexamined Patent Application Publication No. 2008-231995 has been known, for example. This estimating method is based on the assumption that a state change of a mixture gas in a compression stroke of the internal combustion engine is a polytropic change, and that a heat capacity ratio of the mixture gas varies depending on its composition. More specifically, an in-cylinder pressure sensor detects in-cylinder pressures P1, P2 at two predetermined crank angles CA1, CA2 in the compression stroke, and a heat capacity ratio κ of the mixture gas is calculated from the in-cylinder pressures P1, P2 and cylinder volumes V1, V2 corresponding to the crank angles CA1, CA2 in accordance with the following formula:κ=log(P1/P2)/log(V2/V1).
Then, the EGR ratio is calculated based on the calculated heat capacity ratio κ by referring to a predetermined table in which a relationship between the heat capacity ratio and the EGR ratio (EGR gas concentration) is defined.